HCS12 datasheet(340/452 Pages) FREESCALE

340 / 452 page

S12CPUV2 Reference Manual, Rev. 4.0

340

Freescale Semiconductor

The knowledge base can be developed by an application expert without any

microcontroller programming experience. Membership functions are simply

expressions of the expert’s understanding of the linguistic terms that

describe the system to be controlled. Rules are ordinary language

statements that describe the actions a human expert would take to solve the

application problem.

Rules and membership functions can be reduced to relatively simple data

structures (the knowledge base) stored in non-volatile memory. A fuzzy

inference kernel can be written by a programmer who does not know how

the application system works. The only thing the programmer needs to do

with knowledge base information is store it in the memory locations used by

the kernel.

One execution pass through the fuzzy inference kernel generates system

output signals in response to current input conditions. The kernel is

executed as often as needed to maintain control. If the kernel is executed

more often than needed, processor bandwidth and power are wasted;

delaying too long between passes can cause the system to get too far out

of control. Choosing a periodic rate for a fuzzy control system is the same

as it would be for a conventional control system.

9.2.1 Fuzzification (MEM)

During the fuzzification step, the current system input values are compared

against stored input membership functions to determine the degree to which

each label of each system input is true. This is accomplished by finding the

y-value for the current input value on a trapezoidal membership function for

each label of each system input. The MEM instruction in the CPU12

performs this calculation for one label of one system input. To perform the

complete fuzzification task for a system, several MEM instructions must be

executed, usually in a program loop structure.

Figure 9-2 shows a system of three input membership functions, one for

each label of the system input. The x-axis of all three membership functions

represents the range of possible values of the system input. The vertical line

through all three membership functions represents a specific system input

value. The y-axis represents degree of truth and varies from completely

false ($00 or 0 percent) to completely true ($FF or 100 percent). The y-value

where the vertical line intersects each of the membership functions, is the

degree to which the current input value matches the associated label for this

system input. For example, the expression “temperature is warm” is 25

HCS12 Datasheet(PDF) 340 Page - Freescale Semiconductor, Inc